For years, aerospace and defense firms have been worrying about the Baby Boomer bomb, when thousands of their most experienced workers would retire.

About 19% of workers in the industry were 65 or older and eligible for retirement last year, a number that is expected to jump to 40% by 2014, according to the 2010 Aviation Week Workforce Study.

But then the Great Recession occurred and the older workers didn’t leave and no one knows exactly when they will now.

The study found that 33% of those eligible for retirement cited the fact they they were upside down on their 401(k) plans as the reason why they were staying on. As a result, in the last year, the aerospace/defense retirement rate dropped from 5.7% of eligible retirees to 2%.

This has presented the industry with a couple of problems: how and when will these older workers leave and how do they keep their younger employees in the meantime?

The study found that the industry has a 21% attrition rate among its young professionals 35 and under — the workers who will be needed once this Baby Boomer bomb hits.

Companies are looking at new strategies to retain these workers, including moving them through a series of work assignments, said Aviation Week.

“Keeping people excited is our challenge,” Norma Clayton, vice president of learning, training and development at the Boeing Co. told the magazine. “When they get bored, they stop being creative.”

Nonetheless, Aviation Week said that the industry is still hiring nationwide this year although in much-reduced numbers. The industry will bring on about 15,500 net new workers this year, including 1,562 in production. That’s down form 19,000 new hires last year.